The present invention relates to a damping force adjustable fluid pressure shock absorber in which damping force characteristics can be appropriately adjusted.
As a hydraulic shock absorber mounted on a suspension apparatus of a vehicle such as an automobile, there is known a damping force adjustable hydraulic shock absorber in which damping force characteristics can be appropriately adjusted in accordance with road surface conditions, vehicle running conditions, and the like, so as to improve ride comfort and steering stability.
Generally, a damping force adjustable hydraulic shock absorber comprises a sealed cylinder in which oil is contained, a piston rod, and a piston coupled to the piston rod. The piston is slidably fitted in the cylinder so as to divide the inside of the cylinder into two chambers. The piston includes a primary oil passage and a bypass passage through which the two chambers in the cylinder are in communication with each other. A damping force generating mechanism is provided at the primary oil passage. The damping force generating mechanism comprises an orifice, a disk valve and the like. A damping force adjusting valve is provided at the bypass passage. The damping force adjusting valve adjusts the passage area of the bypass passage.
To reduce a damping force, the bypass passage is opened by use of the damping force adjusting valve so as to reduce resistance to an oil flow between the two chambers in the cylinder. To increase a damping force, the bypass passage is closed so as to increase resistance to an oil flow between the two chambers. In this way, damping force characteristics can be appropriately adjusted by opening or closing the damping force adjusting valve.
However, a problem exists in a hydraulic shock absorber in which damping force adjustment relies, as described above, on changing only a passage area of a bypass passage; that is, although it is possible to change damping force characteristics to a large extent in a low piston speed range because in this range a damping force depends on an orifice dimension of an oil passage, it is not possible to change damping force characteristics to a large extent in middle and high piston speed ranges because in these ranges a damping force depends on an opening degree of a damping force generating mechanism (for example, disk valve) provided at a primary oil passage.
With the aim of solving this problem, for example, Japanese Patent Application Public Disclosure 2003-278819 discloses a damping force adjustable hydraulic absorber provided with a pilot type damping force adjusting valve as a damping force generating mechanism on a primary oil passage. In the pilot type damping force adjusting valve, a backpressure chamber (pilot chamber) is formed at the back of a disk valve, and the backpressure chamber is in communication through a fixed orifice with a cylinder chamber of an upstream side of the disk valve, and is also in communication through a flow rate control valve (pilot control valve) with a cylinder chamber of a downstream side of the disk valve.
According to this damping force adjustable hydraulic absorber, the area of the communication passage between the two chambers in the cylinder can be directly adjusted by opening or closing the flow rate control valve, and at the same time, the valve-opening pressure of the disk valve can be changed by changing the pressure in the backpressure chamber by utilizing pressure loss occurring at the flow rate control valve. In this way, it is possible to adjust not only orifice characteristics (in which a damping force is approximately proportional to the square of piston speed) but also valve characteristics (in which a damping force is approximately proportional to piston speed), thereby to widen an adjustable range of damping force.
As a further improved art, for example, Japanese Patent Application Public Disclosure 2006-292092 discloses a damping force adjustable hydraulic shock absorber in which a simple structure is realized by partially sharing an oil flow passage between an extension side and a compression side, and using a single damping force adjusting valve to adjust respective damping forces of the extension and compression sides.
However, the damping force adjustable hydraulic shock absorber disclosed in the above mentioned Japanese Patent Application Public Disclosure 2006-292092 has the following problem. Namely, since a valve body of the damping force adjusting valve (solenoid valve) includes pressure receiving surfaces on its end portion, it is not possible to form a passage through the valve body, through which chambers at both ends of the body could otherwise communicate with each other and thereby provide a pressure balance. Consequently, it is difficult to adequately recover a volume loss caused by a movement of the valve body, as a result of which smooth opening and closing of the valve is liable to be impeded, and responsiveness of the hydraulic shock absorber is liable to deteriorate.